Kw/hrs grid &amp; Solar

Not being an electrical engineer I am curious if kw/hrs grid can be used to determine the requirements for a solar system. For instance over a period of ten months on the grid I used an average of 103 kw/hrs, the highest month being 180 kw/hrs & the lowest being 58 kw/hrs.

Keeping in mind I am on 240V here, if that is relevant, can I use these figures to design a system which just fits my needs without being too stingy or extravagant?

Comments

kWH from your utility is about perfect for working out many of your questions ahead of time.

Now the questions... Is that 103 kWH per day or per 30 days or something else?

Just to give you an idea of typical energy usage:

1,000 WH per day (1 kWH) is good for cabin lighting, 12 volt water pump, laptop computer, cell phone charging3,300 WH per day (3.3 kWH per day or 100 kWH per month) add refrigerator, washing machine, well pump, TV, radio. (about the minimum amount of electricity needed for a "near normal" electric life with lots of conservation. Using other fuels for cooking, heating, hot water.10,000 WH per day (300 kWH per month)--More electric appliances16,000 WH per day (500 kWH per month) -- The bottom range of a "typical" north American home's energy usage33,000 WH per day (1,000 kWH per month) -- The upper range of a "typical" NA home (electric water heating, electric stove, etc.)100,000 WH per day (3,000 kWH per month) -- Heavy A/C usage in Texas / all electric home

1,000 WH to 3,300 WH per day system is a fairly reasonable Do It Yourself system (if you are so inclined). Larger systems you may need to contact a professional.

My take is, the figures are monthly, which is about my consumption average although my monthly values are all close to 100 kWh per month including inverter and battery losses. Appliances refrigerator 50-120 W, TV 65W about 3 hours use, washing machine every other day various wattage, rice cooker 15 minutes 500W, led lights 12W 4 hours, charging phones and tablet negligible, cooking and water heating use LPG. Adding the losses to the actual AC consumption the grid records reveal will be pretty close to what is required. The choice of batteries will influence the losses to some degree, due to inherent efficiency/inefficiencies of differing chemistries. Overall add approximately 1Kwh pre day ballpark, to the high month, this should be close to what's needed. Note. All my water pumping needs are taken care of by an independent system which are for a farm, so greater than average home consumption therfore not included.

To build a system there are other factors including but not limited to, geographic location which need to be considered, having data of typical useage is valuable in establishing a starting point.

What all are you powering with grid supplied power that your usage is so low? My off-grid setup in Baja will use more than 58Kwh. in a typical month when I'm not there. This would consist of the energy efficient refer (obviously not being opened), one led lamp. the backlit weather station monitor, and the clock on the microwave.

The site appears to be saying a 1kw array will produce an average of between about 1.5kwh/day in July to nearly 6kwh/day in best solar month.

Normally, you would size a battery bank to support expected loads for some period of time (eg. 2days), and size a solar array to properly charge the bank. In the worst solar months, sizing big enough may be impractical, so a generator would be expected to provide much of the power.

Then there is charging the battery bank. 5% rate of charge can work for weekend/summer seasonal cabin... 10%-13%+ suggested for full time off grid system (note: The larger the battery bank, the more "minimum" charging current needed):

Decent sun during winter (3 hours+ of sun is not bad).

Now--You can decide your minimum needed power (don't count on a solar array producing 100% of predicted energy every day). For "base loads" (refrigerator, water pump, LED lighting), assume 50% to 65% of predicted output (loads you need to run every day) vs loads that are optional... Irrigation, Fans in summer, clothes washer, electric cooker/tea kettle, etc... Those you run when you have lots of "excess power" available (or are OK with running a backup genset during bad weather/heavy power needs).

So that is the basic calculation and you can now size the system, pick hardware, and cost the system.

And design if 6 kWH per day is really your goal, or if you can spend some money on conservation (more efficient refrigerator, "solar friendly" well pump, etc...

And to give you an idea of what such a system could support... for a 588 AH @ 48 volt battery bank (FLA), the maximum suggested AC inverter (and more or less maximum solar array) would be 5.88 kWatts (1,000 Watts per 100 AH @ 48 volt battery bank). Depending on your needs (surge loads for well pump, shop tools, etc.), the AC inverter range would be something like 2,940 to 5,880 Watts.

Toss the bottom three months (assume genset/conservation during winter), the next minimum would be 3.82 hours of sun per day. The generated power would be (nominal):

Energy usage is a highly personal set of choices (what may work for me, may not work for your needs)... The above is a starting point for you to play with the numbers. Typically I like to be "on the money" for battery bank design (Goldilock--Not too big, not too small). Solar arrays are historically cheap, so a larger solar array is going to both last many decades (20-40+ years for good panels) and probably save you a lot of genset run time (fuel and maintenance).

And, it will "keep the battery bank happy" (don't discharge below 50% state of charge, >90% State of charge once or twice per week--Mostly from solar).

Are they saying here I will need the equivalent to 1700 watts in solar panels? Plus batteries for storage of course?

This looks like it's giving you what the solar array will produce, This number can be used for Grid tied systems. but for off grid system there is roughly 50% losses (look at Bills figures above) and you want the system to reach fully charged a couple times a week.

It's up to you to look at the energy production profile and match it to your energy use profile.

Where I am, I use as much as 18 kWhs in the summer running an air conditioners and electric water heating (not suggesting this!) It's no problem with my 4000 watt array, as we get a lot of sun with our heat. In the fall we can have long cloudy periods which are much more troublesome. Going a week without direct sunlight is a challenge when the fridge is eating 1-1.5 kWhs a day.

It's all a little 'touchy feely' balancing act.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol

Take advantage of "opportunity loads" Mid day, on into the afternoon, you can do laundry, vacuum, run power tools and other motorized chores using the potential power available from your array that would otherwise be wasted.

Thanks Bill. Difficult to go all in as I do have the option to get connected to the grid at probably around 5k all up. Any decent solar with backup is going to cost at least double and I can't see a payback within my lifetime. Still as they say you can't take it with you.

Generally, if you have utility power available, then get a generator setup... Propane or natural gas if you have it, diesel or gasoline otherwise (and a way to "burn" the fuel every 2-3 years for diesel, every ~1 year with gasoline/petrol--Use fuel stabilizer for both).

Gensets only burn fuel if the power is out--Very little maintenance costs when they are not running. A small genset (like a Honda eu2000i or equivalent ~1,600 Watt genset) will run around 4-9+ hours on a gallon of fuel. 10 gallons of fuel will give you 5-10 days of useful power (even enough for a refrigerator). If you are a belt and suspenders guy--Get two gensets.

You can get larger gensets, but now you are tied to a fuel hog (5+ gallons of fuel per day)--You need to find a service station to get more fuel (if they have power and fuel) and transport it home (as well as more fuel to recycle every 1-3 years).

A 1,000 WH per day solar power system system can keep you safe (LED lighting, laptop, RV water pump).

And a 3,300 WH per day system will give you a refrigerator, well pump, and washing machine--Nice end of the world system (not too expensive, small enough for you to install and maintain).

But solar power systems/battery powered--Batteries are dying from the day they are installed (3-8 years typical, depending on brand/quality/temperature of battery shed). Solar/Battery systems really are hard to justify as "backup power" systems that may only be used a few weeks a year.

Bill here in NZ most wouldn't even think about end of the world - they just connect and expect the grid to be there forever. But in US is different there I guess what with rising political divisions etc.

This should be a different thread perhaps but maybe it's possible to build up to your own needs over time rather than to get it all at once. For instance I have an 85 Amp/hour battery and a 160 watt PV array and yet that battery takes 8 hours to charge (no load) from 50% discharged to full. There is something odd going on. But I will buy a new 100 Amp/hour battery and find out how long that takes to charge compared to the older one. I need more storage anyway, but along the way I will discover what is an acceptable charge rate and what's not. Going forward I can upgrade either battery or panels as required until I get to a system I can live with.

The "good old flooded cell lead acid"... It works best if you plan on discharging about 25% per day... That way, the next day you can get it fully recharged (or close to it at 10% to 13% rate of charge).

The Li Ion (LiFePO4 -- Lithium Iron Phosphate is a "safer" Li Ion chemistry in general) and (some?) of the AGM batteries can recharge much faster that standard FLA batteries... Fullriver batteries can bulk much higher rate of charge (of course, that means a larger solar array for quick charging).

I would suggest as a starting point is 2x 6 volt @ ~200 AH FLA "golf cart" batteries--At least, in the US, these are very popular, relatively cheap, and pretty forgiving. Put 2 of them in series for a 12 volt @ ~200 AH battery bank.

Using the off grid home rules of thumb, this would give you something like:

Get a good glass hydrometer (for flooded lead acid batteries) and, if you can justify it, an AC/DC Current Clamp DMM

This would give you a nice system that will produce a useful amount of power, but not break the bank. Assuming 6 volt @ 200 AH "golf cart" batteries are popular in your area (about USD $100 each in USA).

The "end of the world for us" (ignoring earthquakes and fires for California; Hurricanes for eastern US) has been our state government doing things for the "children" (our last deregulation in 2000 just about bankrupted our two major utilities and had us on rotating outages for a few days before they "saw the light"). Looks like we are about to do something similar with "going 100% green utility power" (like in Australia where one or two states went so "green" that they cannot keep companies with stable power--From what I have read).

There's a lot to be said for starting small. You get real-world experience with a live system, and if you end up murdering your first bank (wouldn't be all that unusual), it's not a huge hit. It can also help sort the loads you really can't/won't live without from those you can.

My take on the start small system is yes, but in the case of 230V AC use a 24V nominal or higher, rather than 12V, rationale is the conversion ratio 20:1? for every amp at 230V there is a need for approximately 20A on the DC side, with a higher voltage the current is reduced, additionally its it's easier to expand with a higher voltage rather than being stuck with a restrictive imperative of a 12 V nominal. system. My opinions others may differ.

The refrigerator, in most cases, is what determines the system size, there are DC compressor refrigerators which are extremely efficient, albeit at a cost, commercially available inverter refrigerators are available which can run on small inverters due to lack of surge/inrush current, my Panasonic NR-BW 415, which is available in N.Z., can run on a 300W inverter, which is what I do using an automatic transfer switch, programming my main inverter to go to sleep overnight thus transferring to the smaller inverter, saving the main inverter self consumption load which exceeds the refrigerator consumption overnight, one has to be creative when grid power is not available

@mcgivor You are right the Panasonic is available here at $NZ1350 / $US910. Are you powering this from a PSW inverter? It might be worth considering as I do have a 600 watt PSW. So would have 300 spare for, say, a laptop. Not sure if that could done.

In addition $1000 plus is what I could be expecting for the cost of additional solar just to power the fridge I have now.

@mcgivor You are right the Panasonic is available here at $NZ1350 / $US910. Are you powering this from a PSW inverter? It might be worth considering as I do have a 600 watt PSW. So would have 300 spare for, say, a laptop. Not sure if that could done.

In addition $1000 plus is what I could be expecting for the cost of additional solar just to power the fridge I have now.

While it doesn't require a large inverter, it will require more solar than a 160 watt array is likely to produce.

Also It looked like you intend to add a 100ah battery to a 85ah battery? In general only batteries of the same age and size 'play well together'.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol

@mcgivor You are right the Panasonic is available here at $NZ1350 / $US910. Are you powering this from a PSW inverter? It might be worth considering as I do have a 600 watt PSW. So would have 300 spare for, say, a laptop. Not sure if that could done.

In addition $1000 plus is what I could be expecting for the cost of additional solar just to power the fridge I have now.

While it doesn't require a large inverter, it will require more solar than a 160 watt array is likely to produce.

Also It looked like you intend to add a 100ah battery to a 85ah battery? In general only batteries of the same age and size 'play well together'.

True, am presently 'freshening' charge 2 * new 6v 220ah Crowns. Next, I plan to purchase the inverter fridge and allocate 100% of the solar to discover if or not that will be sufficient.

Trial & error etc.

But prior to the delivery of the fridge I will use the new setup to see how it handles my other than fridge requirements.

Update: I finally settled on a digital inverter fridge / freezer Samsung SR255MLS. It is rated at 269 kWh per year. This has been running off my present setup (310W, 220 Ah, 600 PSW inverter). Cheating a bit as my DC is coming off the grid. But still, quite impressive. I did have this fridge / freezer running completely off solar charge but after 24 hours put it back on. Potentially it would have been good for 36 hours but I am babying my batteries.